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Breathe, Neo. I’ve been running a marathon lately to cover all
the major players that may provide viable alternatives to fossil
fuels this century. Even though I have not exhausted all
possibilities, or covered each topic exhaustively, Iam
exhausted. So in this post, I will provide a recap of all the
schemes discussed thus far, in matrix form. Then Do the Math will
shift its focus to more of the “what next” part of the message.

The primary “mission” of late has been to sort possible future
energy resources into boxes labeled “abundant,” “potent” (able to
support something like a quarter of our present demand if fully
developed), and “niche,” which is a polite way to say puny. In
the process, I have clarified in my mind that a significant
contributor to my concerns about future energy scarcity is not
the simple quantitative scorecard. After all, if it were that
easy, we’d be rocking along with a collective consensus about our
path forward. Some comments have asked: “If we forget about
trying to meet our total demand with one source, could we meet
our demand if we add them all up?” Absolutely. In fact, the
abundant sources technically need no other complement. So on the
abundance score alone, we’re done at solar, for instance. But
it’s not that simple, unfortunately. While the quantitative
abundance of a resource is key, many other practical concerns
enter the fray when trying to anticipate long-term prospects and
challenges—usually making up the bulk of the words in prior
posts.

For example, it does not much matter that Titan has enormous
pools of methane unprotected by any army (that we know of!). The
gigantic scale of this resource makes our Earthly fossil fuel
allocation a mere speck. But so what? Practical considerations
mean we will never grab this
energy store. Likewise, some of our terrestrial sources of
energy are super-abundant, but just a pain in the butt to access
or put to practical use.

In this post, we will summarize the ins and outs of the various
prospects. Interpretation will come later. For now, let’s just
wrap it all up together.

The Matrix

Would you like to know what the matrix is? Okay. I’ll tell
you—in a bit. For each of the major energy hopefuls I have
discussed on Do the Math, I characterize their various
attributes in a three-tier classification: adequate
(green); marginal
(yellow); or insufficient
(red)—possibly a
showstopper. The scheme is qualitative, and I am sure some will
disagree with my assignments. Before I go any further, let me
say that I will not entertain comments griping about why I made
a certain square the color I did. I won’t have time to respond
at that level, given that there are 200 colored boxes in the
matrix. But the beauty is, you can change the matrix any
way you see fit and make your own custom version. Go buy
some crayons today!. The matrix I’ve created is not without its
biases and subjectivity. Whose would be?

Okay, I’ll keep the suspense going a bit by describing the
fields.

Abundance: This is essentially the “abundant,”
“potent,” and “niche” classification scheme reflected in the
preceding posts. Green
means that the resource can in principle produce far more power
than we currently use and keep it up for many centuries.
Red means a bit-player at
best. Yellow is the stuff
that can be useful, but is incapable of carrying the full
load—not that we require everything to do this. We can tolerate
a mix of of items, but will not get far by mixing reds
together.

Difficulty: This field tries to capture the
degree to which a resource brings with it large technical
challenges. How many PhDs does it take to run the plant? How
painful is it to maintain or keep churning? This one might
translate into economic terms: difficult is another term for
expensive.

Intermittency: Green if rock-steady or there whenever
we need it. If the availability is beyond our control, then it
gets a yellow at least. The possibility of going without for a
few days earns a red.

Demonstrated: I don’t mean on paper, and I
don’t mean a prototype that exhibits some of the technology. To
be green, this has to be
commercially available today, and providing useful energy.

Electricity: Can the technology produce
electricity? Most of the time, the answer is yes. Sometimes it
would make no sense to try. Other times, it is seriously
impractical.

Heat: Can the resource produce direct heat?
Yellow if only through electric means.

Transport: Does the technology relieve the
liquid fuels crunch? Anything that makes electricity can power
an electric car, earning a yellow score. Liquid fuels are
green. Some may get tired
of the broken record in the descriptions that follow that a
particular resource does not help transportation, wanting to
shout “electric cars, fool” every time I say it. But our
large-scale migration to electric cars is not in the bag. They
may remain too expensive to be widely adopted. Meanwhile, this
does not help air travel or heavy transport.

Acceptance: Is public opinion (I can really
only judge U.S. attitudes) favorable to this method? Will there
likely be resistance—whether justified or not?

Backyard?: Is this something that can be done
domestically, in one’s backyard or small property, managed by
the individual?

Efficiency: Over 50% gets the green. Below about 10% gets
red. It’s not the most
important of criteria, as the abundance score incorporates
efficiency expectations. But we will always view low efficiency
negatively.

Yellow boxes tend to deserve explanation. It is usually clear
why something would swing red or green, but yellow often has
several things tugging at it. If green boxes are given a +1 score, yellow boxes zero, and
red boxes −1, adding the boxes with equal weight
yields the scores on the right, by which measure the table is
sorted: best to worst. The only place I cheated was to give D-D
fusion a −2 for difficulty. It’s the hardest thing on the list,
given our decades of massive effort invested to date on D-T
fusion, while D-D is too hard to even attempt.

Now, equal weighting on all ten criteria is boneheaded. But the
assessment is imprecise enough not to warrant a more elaborate
weighting scheme. I do not stand firm behind the order that
results, and am half-tempted to monkey with weighting schemes
until a more preferred order emerges. But I would be cooking
the books to further match my preferences. Feel free to weight
any way you see fit, and change anything else while you’re at
it. Just remember. No griping.

Fossil Fuels, Compared

Note that conventional fossil fuels,
matrixed-out above, score green in almost every
category, except—unfortunately—abundance. The
efficiency is high for direct heating (most often natural gas),
and middling for electricity or transport. Coal gets no points
for transportation, and natural gas is of limited use here
(although the bus I’m riding as I type this is powered by
natural gas, so I can’t entirely nix its transportation
capability). All things considered, all of the fossil
fuels get a score of 7 or 8. Note the striking
gap we face between fossil fuels and their
alternatives, topping out at a score of 5. One might ding the
fossil fuels a point or two for their greenhouse gas
contributions, closing the gap a bit. None of the options in
the alternatives matrix are intrinsic carbon emitters.

Quick Lessons

Looking at some of the main trends, very few
options are both abundant and easy. Solar PV
and solar thermal qualify. A similar exclusion principle often
holds for abundant and demonstrated/available. There is a
reason why folks (myself included) like solar.

Intermittency mainly plagues
solar and wind resources,
with mild inconvenience appearing for many of the natural
sources.

Electricity is easy to produce. We have loads
of ways to do it, and are likely to pick the easiest/cheapest.
We won’t necessarily get far down the list if we’re covered by
things at the top end (assuming my rankings have any validity
and some economic correlation).

Transport is hard. Concerns over peak oil played a
huge role in making me sit up to pay attention to our
energy challenges. Electric cars are the most obvious way out,
but don’t do much for heavy shipping by land or sea, and leave
airplanes on the ground.

Few things face serious barriers to
acceptance: especially when energy scarcity is
at stake.

A few options are available for the
homestead. A passive solar home with PV
panels, wind, and some method to produce liquid fuels on site
would be a dream come true. Here’s hoping for artificial
photosynthesis!

The missing category here is cost, although
difficulty may be an imperfect proxy. As a result, some of the
high-scoring options may more be costly than we’d like.
Actually, some of the lowest-scoring options are the costliest!
If you’re expecting that we’ll replace fossil fuels
and do it on the cheap, you might as well learn to
bawl on the floor kicking and pounding your fists, tears
streaming. This is our predicament. We have to buck up and deal
with it, somehow.

Individual Discussion

For each topic, the link at the beginning points to a more
complete discussion on Do the Math. Here, I just briefly
characterize each resource in relation to the matrix criteria.

Solar PV: Covering only 0.5% of land area with 15%
efficient PV panels provides the annual energy needs of our
society, qualifying solar PV as abundant. It’s not terribly
difficult to produce; silicon is the most abundant element in
Earth’s crust, and PV panels are being produced globally at 25
GW peak capacity per year (translating to 5 GW of average power
added per year). Intermittency is the Achilles Heel of solar
PV, requiring storage solutions if adopted at large scale.
Solar PV produces electricity directly, which could be
converted to heat or transport. Most people do not object to
solar PV on rooftops or over parking areas, or even in open
spaces (especially desert). I’ve got some on my garage roof as
we speak (with storage), so they’re well-suited to individual
operation/maintenance. Clocking in at an efficiency of 15%,
don’t expect PV to vastly exceed this ballpark.

Solar Thermal: Achieving comparable efficiency to PV, but
using more land area, generating electricity from concentrated
solar thermal energy automatically fits in the abundant
category—though somewhat more regionally constrained. It’s
relatively low-tech: shiny curved mirrors tracking on (often)
one axis, heating oil or other fluid to run a plain-old heat
engine. Intermittency can be mitigated by storing thermal
energy, perhaps even for a few days. Because a standard
heat-engine follows, fossil fuels can supplement in lean times
using the same back-end. A number of plants are already in
operation, producing cost-competitive electricity—and heat if
anyone cares. As with so many of the alternatives,
transportation is not directly aided. Public acceptance is no
worse than for PV, etc. But don’t expect your own personal
solar thermal electricity plant.

Solar Heating: On a smaller scale, heat collected directly
from the sun can provide domestic hot water and home heating.
In the latter case, it can be as simple as a south-facing
window. Capturing and using solar heat effectively is not
particularly difficult, coming down to plumbing, insulation,
and ventilation control. Technically, it might be abundant, but
since it is usually restricted to building footprints (roof,
windows), I take it down a notch. There will be lean days, but
my friends in Maine do not complain about their solar heating
comfort (with occasional propane backup). Solar heating is
useless for electricity or transport, but has no difficulty
being accepted and almost by definition is a backyard-ready
technology.

Hydroelectric: We have seen that super-efficient
hydroelectric is doomed to remain a small player (in the rubric
that we maintain today’s energy consumption levels). It’s the
low-hanging fruit of the renewable world, and has therefore
already seen large-scale development. It has seasonal
intermittency (typical capacity factor for a hydro plant is
40%), does not directly provide heat or transport, and can only
rarely be implemented personally, at home. Acceptance is fairly
high, although silting and associated dangers—together with
habitat destruction—do cause some opposition to expanded
hydroelectricity.

Biofuels from Algae: I was somewhat surprised to see this
entry rank as highly as it did in my admittedly unsophisticated
scoring scheme. Because it captures solar energy—even at <
5% efficiency—the potential scale is automatically enormous.
But it’s not easy, at present. Dealing with slime will bring
challenges of keeping the plumbing clean, possible infection in
a genetic arms race with evolving viruses, contamination by
other species, etc. At present, we don’t have that magic algal
sample that secretes the fuels we want. Heady talk of genetic
engineering pledges to solve these problems, but we’re simply
not there yet and cannot say for sure that we will get there.
Otherwise, the ability to provide transportation fuel is the
big draw. Heat may also be efficiently produced, though
electricity would represent a misallocation of liquid fuel. Can
it be done in the backyard? I could imagine a slime pond in the
yard, but care and feeding and refining the product may be
prohibitively difficult.

Geothermal Electricity: This option makes sense primarily
at geological hotspots, which are rare. It will not scale to be
a significant part of our entire energy mix. Aside from this,
it is relatively easy, steady, and well-demonstrated in many
locations. It can provide electricity, and obviously direct
heat—although far from heat demand, generally. It provides no
direct help on transportation. Objections are slim to
non-existent. I don’t think houses tend to be built on the
hotspots, so don’t look for it in a backyard near you.

Wind: Wind is a sensible option that I imagined would float
higher in the list than it did. It’s neither abundant nor
scarce, being one of those options that can provide a
considerable fraction of our present needs under large-scale
development. It’s pretty straightforward, reasonably efficient,
and demonstrated the world over in large farms. The biggest
downside is its intermittency. It will not be unusual to have a
few days in a row with little or no regional input. Like so
many other things, electricity is naturally produced, while
heat and transport is only available via electricity. I sense
that objections to wind are more serious than for many other
alternatives. Windmills are noisy and tend to be located in
prominent places (ridge-tops) where they are extremely visible
and scenery-altering. You can’t hide wind in a bowl, or you end
up hiding from the wind at the same time. Another built-in
conflict emerges on wind-rich coastlines, where many like to
take in unspoiled scenery. Small-scale wind is viable in your
own backyard.

Artificial Photosynthesis: A very appealing future prospect
for me is artificial photosynthesis, combining the abundance of
direct solar with the self-storing flexibility of liquid fuel.
Intermittency is thus eliminated to the extent that annual
production meets demand: storage of a liquid fuel for many
months is possible. The dream result of a panel sitting on your
roof that drips liquid fuel could provide both heating and
transportation fuel. In a pinch, one could also produce
electricity this way, but what a waste of precious liquid fuel,
when we have so many other ways to make electricity! The catch
is that it doesn’t exist yet, that it may never exist, and that
feeding it the right ingredients and processing/refining the
fuel may eliminate the backyard angle. Still, we all have to
have something to gush over. For some, it’s thorium
and for others it’s fusion, etc. This one excites me by its
potential to satisfy so many purposes.

Tidal Power: Restricted to select coastal locations, tidal
will never be a large contributor on the global scale. The
resource is intermittent on daily and monthly scales, but in a
wholly predictable manner. Extracting tidal energy is not
terribly hard—sharing technology with similarly efficient
hydroelectric installations—and has been demonstrated in a
number of locations around the world. It’s another electricity
technique, with no direct offering of heat or transportation.
No unusual level of societal objection exists, to my knowledge,
but it’s not something you will erect in your backyard and
expect to get much out of it.

Conventional
Fission: Using conventional uranium reactors and
conventional mining practices, nuclear fission does not have
the legs for a marathon. On the other hand, it is certainly
well-demonstrated, and has no problems with
intermittency—unless we count the fact that it has trouble
being intermittent in the face of variable load.
Compared to other options, nuclear runs a tad on the high-tech
side. By this I mean that design, construction, operation, and
emergency mitigation require more brains and sophistication
than the average energy producer. Nuclear fission directly
produces heat (seldom utilized), and is primarily used to
generate electricity via the standard steam-driven heat engine,
but offers no direct help on transportation. Acceptance is
mixed. Germany plans to phase out its nuclear program even
though they are serious about carbon reduction. No new plants
have been built in the U.S. for over thirty years in part due
to public discomfort. Some of this is irrational fear over
mutant three-eyed fish and the like, but some is genuine
political difficulty relating to the pesky waste problem that
no country has yet solved to satisfaction. Nuclear power is not
possible on a personal scale.

Uranium
Breeder: Extending nuclear fission to be able to use the
140-times more abundant 238U (rather than 0.7%
235U) gives uranium fission the legs to run for at
least centuries if not a few millennia, so abundance issues
disappear. Breeding has been practiced in military reactors,
and indeed some significant fraction of the power in
conventional uranium reactors comes from 238U turned
239Pu. But no commercial power plants have been
built to deliberately access the bulk of uranium, turning it
into plutonium at scale for the purpose of power production.
Public acceptance of breeders will face even stiffer hurdles
because plutonium is more easily separated into bomb material
than is 235U, and the trans-uranic radioactive waste
from this option is nastier than for the conventional cousin.

Thorium
Breeder: Thorium is more abundant than uranium, and only
comes in one flavor naturally, so that abundance is not an
issue. Like all reactors, thorium reactors fall into the
high-tech camp, and include new challenges (e.g., liquid
sodium) that conventional reactors have not faced. There have
been a few instances of small-scale demonstration, but nothing
in the commercial realm, so that we’re probably a few decades
away from being able to bring thorium online. Public reaction
will be likely be similar to that for conventional nuclear: not
a show stopper, but some resistance on similar grounds. It is
not clear whether the newfangled aspect of thorium will be
greeted with suspicion or with an embrace. Though also a
breeding technology (making fissile 233U from
232Th), the proliferation aspect is severely
diminished for thorium due to highly radioactive
232U by-product and virtually no easily separable
plutonium. Of the future nuclear prospects, I am most
optimistic about this one—although it’s no nirvana to me.

Geothermal Heating with Depletion: A vast store of thermal
energy sits in the crust, locked in the rock and moving slowly
outward. Being the impatient lot that we are, we could drill
down and grab the energy out of the rock on our own schedule,
effectively mining heat as a one-time resource. In the absence
of water flow to convect heat around, dry rock will deplete its
heat within 5–10 meters of the borehole in a matter of a few
years, requiring another hole 10 meters away from the first,
and so on and so on. I classify this as moderately difficult,
requiring a never-ending large-scale drilling operation across
the land. The temperatures are pretty marginal for running heat
engines to make electricity with any respectable efficiency
(especially given so many easier options for electricity), but
at least the thermal resource will not suffer intermittency
problems during the time the hole is still useful. Given its
inconvenience (kilometers of drilling), I do not know if
examples abound of people having tried this for the
purpose of providing heat in arbitrary (not geologically
hot) areas. Public acceptance may be less than lukewarm given
the scale of drilling involved, dealing with tailings and
possibly groundwater contamination issues on a sizable scale.
While such a hole could fit in a backyard, it would be far more
practical to use the heat for clusters of buildings rather than
for just one—given the amount of effort that goes into each
hole (and considering short-term lifetime of each hole). I gave
this technique high marks for efficiency if used for heat, but
it would drop to reddish-yellow if we tried to use this
resource for electricity.

Geothermal Heating, Steady State: If we turn our noses up
at depletion-based geothermal heat, steady state offers far
less total potential, coming to about 10 TW of flow if summed
across all land. And to access temperatures hot enough to be
useful for heating purposes, we’re talking about boreholes at
least 1 km deep. It is tremendously challenging to cover any
significant fraction of land area with thermal collectors 1 km
deep. So I am probably being too generous to color this one
yellow for the abundance factor. That’s okay, because I’m
hitting it hard enough on the other counts. To gather enough
steady-flow heat to provide for a normal U.S. home’s heat, the
collection network would have to span a square 200 m on a side
at depth, which seems nightmarish to me. But at least depletion
would not be an issue in this circumstance. Otherwise, this
category shares similar markings and rationale as the depletion
scenario.

Biofuels from Crops: We’ve seen that corn ethanol is a
loser of a scheme on energy grounds, although sugar cane and
vegetable oils fare better. But these compete with food
production and arable land availability, so biofuels from crops
can only graduate from “niche” to “potent” in the context of
plant waste or cellulosic conversion. I have thus split the
abundance and demonstration in two: food crop energy is
demonstrated but severely constrained in scale. Celluosic
matter becomes a potent source, but undemonstrated (perhaps
this should even be red). I do not label the prospect as an
easy one, because growing and harvesting annual crops on a
relevant scale constitutes a massive, perpetual job. If
exploiting fossil fuels is akin to spending our inheritance,
growing and harvesting our energy on an annual basis is like
getting a real job—a real hard job. The main benefit
of biofuels from crops is that we get a liquid fuel out of
it—so hard to come by via other alternatives. Public acceptance
hinges on competition with food or just land in general.
Scoring only about 1% efficient at raking in solar energy, this
option requires commandeering massive tracts of land. A
small-time farmer may make useful amounts of fuel for
themselves in their back “yard,” if refining does not create a
bottleneck.

Ocean Thermal: The ocean thermal resource uses the 20–30°C
temperature difference between the deep ocean (a few hundred
meters down) and its surface to drive a ridiculously
low-efficiency heat engine. The heat content is not useful for
warming any home (it’s not hot). But all the same, it’s a vast
resource due to the sheer area of the solar collector. Large
plants out at sea will be difficult to access and maintain, and
transmitting power to land is no picnic either. The resource
suffers seasonal intermittency at mid-latitudes, but let’s
imagine putting these things all in the tropics to get around
this. Sound hard, you say? Well yeah! That’s part of what makes
ocean thermal difficult! No relevant/commercial scale
demonstration exists. Like so many others, this is electricity
only (and this time, far from demand). Probably nobody cares
what we put to sea: out of sight, out of mind. Ocean thermal is
not a backyard solution!

Ocean Currents: Large-scale oceanic currents are slower
than wind by about a factor of ten, giving a kilogram of
current 1000 times less power than a kilogram of wind. Water
density makes up the difference to make ocean current
comparable to wind in terms of power per rotor area. Not all
the ocean has currents as high as 1 m/s, so I put the total
abundance in the same category as wind. Maybe accessing a
thicker column of water than we can for wind should bump ocean
currents up a bit, but the currents are relatively confined to
surfaces. But why dunk a windmill underwater where it’s far
from demand and difficult to access and maintain, when a
comparable power can be had in dry air? So I classify this as
difficult. On the plus side, the current should be rock solid,
eliminating intermittency worries, unlike wind. Still, not one
bit of our electricity mix comes from ocean currents at
present, so it cannot be said to have been meaningfully
demonstrated. For the remaining categories: it’s electricity
only; who cares what’s underwater; and no backyard opportunity.

Ocean Waves: While they seem strong and ever-present, waves
are a linear-collection phenomenon, and not an areal
phenomenon. So there really isn’t that much arriving at shores
all around the world (a few TW at best). It’s not particularly
difficult to turn wave motion into useful electricity at high
efficiency, and the proximity to land will make access,
maintenance, and transmission far less worrisome than for the
previous two cases. There will be some
intermittency—largely seasonal— as storms and lulls come and
go. I’ve seen a diverse array of prototype concepts, and a few
are being tested at commercial scale. So this is further along
then the previous two oceanic sources, but not so much as to
get the green light. There will be moderate push-back from
people whose ocean views are spoiled, or who benefit from
natural wave energy hitting the coast. There are no waves in my
backyard, and I hope to keep it this way!

D-T
Fusion: The easier of the two fusion options, involving
deuterium and tritium, represents a longstanding goal under
active development for the last 60 years. The well-funded
international effort, ITER, plans to accomplish a 480 second
pulse of 500 MW power by 2026. This defines the pinnacle of
hard. Fusion brings with it numerous
advantages: enormous power density; moderate radioactive waste
products (an advantage?!); abundant deuterium (though tritium
is zilch); and surplus helium to liven up children’s parties.
Fusion would have no intermittency issues, can directly produce
heat (and derivative electricity), but like the others does not
directly address transportation. The non-existent tritium can
be knocked out of lithium with neutrons, and even through we
are not awash in lithium, we have enough to last many thousands
of years. We might expect some public opposition to D-T fusion
due to the necessary neutron flux and associated radioactivity.
Fusion is the highest-tech energy we can envision at present,
requiring a team of well-educated scientists/technicians to
run—meaning don’t plan on building one in your backyard, unless
you can afford to have some staff PhDs on hand.

D-D
Fusion: Replacing tritium with deuterium means abundance of
materials is no concern whatsoever for many billions of years.
As a trade, it’s substantially harder than D-T fusion (or we
would not even consider D-T). D-D fusion requires higher
temperatures, making confinement that much more difficult. It
is for this reason that I gave D-D fusion a −2 score for
difficulty. It’s not something we should rely upon to get us
out of the impending energy pinch, which is my primary
motivation.

End of an Era

Not only does this conclude the end of the phase on Do the Math
where we evaluate the quantitative and qualitative benefits and
challenges of alternatives to fossil fuels, it also points to
the fact that we face the end of a golden era of energy. Sure,
we managed to make scientific and cultural progress based on
energy from animals, slaves, and firewood prior to discovering
the fossil fuels. But it was in unlocking our one-time
inheritance that we really came into our own. Soon, we will see
a yearly decrease in our trust fund dividend, forcing us to
either adapt to less or try to fill the gap with replacements.
What this post and the series preceding it demonstrates is that
we do not have a delightful menu from which to select our
future. Most of the options leave a bad taste of one form or
the other.

When I first approached the subject of energy in our society, I
expected to develop a picture in my mind of our grandiose
future, full of alternative energy sources like solar, wind,
nuclear, biofuels, geothermal, tidal, etc. What I got instead
was something like this matrix: full of inadequacies,
difficulties, and show-stoppers. Our success at managing the
transition away from fossil fuels while maintaining our current
standard of living is far from guaranteed. If such success is
our goal, we should realize the scale of the challenge and
buckle down now while we still have the resources to develop a
costly new infrastructure. Otherwise we get behind the curve,
possibly facing unfamiliar chaos, loss of economic confidence,
resource wars, and the unforgiving Energy Trap. The other
controlled option is to deliberately adjust our lives to
require fewer resources, preferably abandoning the growth
paradigm at the same time. Can we manage a calm, orderly exit
from the building? In either case, the first step is to agree
that the building is in trouble. Techno-optimism keeps us from
even agreeing on that.